NAPLEX 2025 – 400 Free Practice Questions to Pass the Exam

Antimetabolites, like MTX, inhibit the production of nucleotides which are essential for DNA synthesis. This means that they are most effective during the S phase of the cell cycle when DNA is being replicated. In contrast, during G1 phase, the cell prepares for DNA replication and the production of nucleotides is not as crucial, making option A incorrect. Similarly, during G2 phase, the cell is preparing for cell division and not actively replicating DNA, so this option is also incorrect. Finally, option D, which suggests that the effect of MTX is not dependent on the cell cycle, is also incorrect as MTX specifically targets cells in the S phase. Therefore, the most accurate answer is B.